This is a convenient measure of the tendency for any chemical process to occur spontaneously, depending on how negative DG is. Table 1 shows the dependence of G on the signs of enthalpy and entropy.

As with enthalpy there is no way to measure absolute values for Gibbs free energies, consequently we define the Gibbs free energy for each of the elements at 1 atm and 298K to be equal to zero; free energies for molecules under these conditions are referred to as standard free energies. Therefore, by analogy to the enthalpy of formation, Gibbs free energy of formation is as follows;

DG∞ f = DH∞


+ + Positive at low temperature, negative at high temperature. Reaction spontaneous in the forward direction at high temperature, and in the reverse at low temperatures

+ - Positive at all temperatures. Reactions spontaneous in reverse direction at all temperatures

- + Negative at all temperatures. Spontaneous in forward direction at all temperatures

- - Negative at low temperature, positive at high temperature. Spontaneous at low temperature, reverse at high temperature

There are a number of ways in which the expression for DG may be written, and these are useful when considering how temperature and pressure affect this parameter.